The reply is the typical thing you see from the Belgium Royals, a pre-made card and photo (although, they don't always send a photo).
The message is in French. Translated the message reads:
Royal Palace of Brussels
The wishes that you were kind enough to send to Her Royal Highness Princess Elisabeth on the occasion of her 16th birthday, have given her great pleasure.
Their Majesties the King and the Queen thank you with all their heart for this beautiful gesture.
The photo is part of a set released for Princess Elisabeth's birthday. No details on the photo. But given the green glass door and plants, I assume this was taken at the Royal Greenhouses in Laeken.
The Royal Greenhouses in Laeken, is a favorite location for the Belgium Royal Family. The Greenhouses are located in the Royal Park at the Castle of Laeken, which is the Royal Family's home. (The Royal Palace of Brussels is just a working palace.)